1. Field of the Invention
The invention relates to a method for recovering metal values from catalyst materials, and more particularly to catalyst materials which have been used in hydrocarbon refining processes, comprising compounds of cobalt, nickel, molybdenum and vanadium on supports which contain aluminum oxide.
2. Description of the Invention
Catalytic hydrorefining, that is, catalyzed reaction under conditions of elevated temperature and pressure in the presence of hydrogen, has become a most useful part of hydrocarbon refining, facilitating the conversion of shale oils, petroleum distillates and residua to various more valuable products. Hydrorefining units are typically capable of prolonged on-stream operation, but suffer from progressively lessened efficiency during operation due to deactivation of the catalyst.
Hydrorefining catalysts are usually comprised of a porous inorganic support, normally one or more refractory oxides such as alumina, silica-alumina composite, amorphous or crystalline aluminosilicates, and the like, impregnated with compounds of a Group VI-B metal (e.g., molybdenum) and a Group VIII metal (e.g., cobalt and/or nickel). Also utilized in such catalysts, but less frequently, are tungsten, titanium, magnesium and several other elements.
A common hydrorefining catalyst, for example, comprises up to about 20 percent by weight molybdenum (calculated as MoO.sub.3) and about 2 to 6 percent by weight nickel or cobalt (calculated as NiO or CoO), on a particulate support of porous alumina. This type of catalyst is customarily utilized for the hydrogenation of crude oil distillates. The cobalt-containing catalyst also has found wide utility in the hydrodesulfurization treatment of petroleum residual oils, which contain impurities such as sulfur, nickel and vanadium at much higher levels than are found in distillates.
Prior to use, the hydrorefining catalyst is "sulfided" by treatment with hydrogen sulfide or compounds which will form hydrogen sulfide in the presence of hydrogen, e.g., mercaptan compounds. As a result of this treatment, the molybdenum and cobalt or nickel is at least partially present on the catalyst as a metal sulfide.
During operation of the hydrorefining process, carbonaceous deposits accumulate on the catalyst particles, thereby inhibiting the catalyst activity. In addition, but at a much slower rate, the metallic impurities nickel and vanadium (as well as lead, iron and other trace elements present in the hydrocarbon feedstock) also form inhibiting deposits on the catalyst. This metal accumulation is, of course, more acute for the treatment of residua, due to the higher impurity levels in such material.
When the catalyst activity has been reduced to an unacceptable level by deposit accumulation, it is customary to restore a large proportion of the previous effectiveness by a regeneration procedure, involving heating the catalyst at elevated temperatures in the presence of an oxygen-containing gas. This rather efficiently removes the carbonaceous deposits, but does not affect the metallic compound deposits. Consequently, after several regenerations, the catalyst must be removed from service and replaced.
It will be appreciated that the metals contained in a spent catalyst, as described, have a substantial value. For a catalyst used in the hydrodesulfurization of residual oils, deposits of as much as about 5 percent by weight nickel (calculated as NiO) and up to about 20 percent by weight vanadium (calculated as V.sub.2 O.sub.5) accumulate on the particles during their useful lifetime. In addition, the original amounts of molybdenum and cobalt or nickel remain in the catalyst particles, although their concentrations are made lower due to the dilution effect of accumulated deposits. Since large quantities of catalyst are used in the refining of petroleum, a number of systems have been developed for recovering the metal values in a commercially useful form.
Most of the reported methods for recovering metal elements from catalyst materials involve leaching with alkaline solutions. Fox et al., in U.S. Pat. No. 3,773,890, suggest roasting calcined catalyst with sodium chloride, which converts vanadium and molybdenum values to a water soluble form. After water leaching, the vanadium and molybdenum solution is separated and the residue is treated with an alkaline solution (such as sodium hydroxide) to dissolve aluminum. The aluminum-containing solution is separated, leaving a residue which contains cobalt and nickel in a concentrated form.
U.S. Pat. No. 4,075,277 to Castagna et al. is directed to the recovery of high purity molybdic acid from catalyst materials. The catalyst is impregnated with an aqueous solution of sodium carbonate and heated to convert molybdenum compounds into sodium molybdate, while avoiding substantial conversion of alumina into a water soluble compound. Molybdenum is then separated from alumina, cobalt and nickel by extraction with hot water.
In U.S. Pat. No. 4,087,510 to Steenken, the recovery of vanadium and molybdenum is accomplished by mixing catalyst with solid alkali metal carbonate, heating to convert sulfur, vanadium and molybdenum into water soluble compounds, and extracting the soluble materials with water.
Toida et al. teach a more complex separation scheme in U.S. Pat. No. 4,145,397, wherein molybdenum, vanadium, cobalt and nickel are recovered. Roasted catalyst is subjected to leaching with a hot caustic alkali solution, which solubilizes most of the vanadium, molybdenum and some of the aluminum. Insoluble residues are treated with a hot acid solution to dissolve most of the cobalt and nickel, some additional aluminum, and a small amount of remaining vanadium and molybdenum. In addition to requiring two types of leaching steps, the lack of specificity in leaching results in a highly complicated series of separations for each leach solution.
An example of metal extraction from catalysts using a salt solution is U.S. Pat. No. 3,567,433 to Gutnikov, in which a hot ammonium carbonate solution is used to dissolve molybdenum, vanadium and nickel. Only molybdenum is substantially completely solubilized by the procedure.
Acid solutions have not found significant use in catalyst metals recovery. Heretofore it has been considered desirable to avoid dissolution of aluminum, insofar as possible, because aluminum is a comparatively low-value component of the catalyst. Since the catalyst contains more aluminum than any other metal, a leach solution which contained proportions of valuable metals and aluminum similar to those of the catalyst would complicate subsequent separation. In addition, aluminum has commonly been considered a nuisance by-product which can be sold, but also requiring a recovery treatment which is quite expensive when compared to its market value.
Aluminum sulfate is, in fact, a highly marketable commodity, both in the form of its solid hydrate and as aqueous solutions, finding a significant use in water treatment and in the paper industry to facilitiate sizing. Solutions for these and other uses typically contain about 7.5 to about 8.5 percent by weight aluminum (calculated as Al.sub.2 O.sub.3), are low in iron content, and contain approximately stoichiometric equivalents of aluminum and sulfate (i.e, a sulfate molarity 1.5 times the molarity of aluminum), with zero or only a slight excess of sulfuric acid permitted. The absence of excess sulfuric acid is also necessary if solid aluminum sulfate hydrate is to be produced by crystallization from the solution.
The commercial production of aluminum sulfate solutions is typically accomplished through the action of sulfuric acid on bauxite, a naturally occurring mineral which has the empirical formula Al.sub.2 O.sub.3.2H.sub.2 O, but is probably a mixture of hydrous aluminum oxides and aluminum hydroxides. Less common substitutes for bauxite are materials such as clays and fly ash, i.e., ash produced by the burning of coal. These substitutes are generally economically unacceptable due to their lower aluminum content and the relatively greater difficulty of extraction over that of bauxite, so remain as potential sources of aluminum should the availability of bauxite become a problem.
An example of an aluminum sulfate production scheme is that of Skay in U.S. Pat. No. 3,079,228 wherein sulfuric acid and a substantial stoichiometric excess of bauxite is introduced into a heated ball mill, and allowed to react during particle grinding. The resulting aluminum sulfate solution is separated, and remaining solids are transferred to a reactor for digestion with a stoichiometric excess of sulfuric acid. Undigestible residues from the reactor are discarded, and the acidic aluminum sulfate solution is recycled to the ball mill for further reaction with bauxite. Disadvantages of the scheme include the multiplicity of process steps and the requirement for two separate time-consuming filtration separations.
Another process, which treats the more reactive forms of aluminum hydroxide, is described in U.S. Pat. No. 3,667,905 to Jennings. Sulfuric acid and the aluminum compound are introduced in stoichiometric proportions into a multi-pass, heat-exchanging reactor. The heat produced by the initial rapid reaction is utilized to promote later, slower reaction completion. To avoid an excess of sulfuric acid in the aluminum sulfate product (caused by incomplete reaction), one is advised to add excess aluminum feed, which necessitates a filtration step for the reactor product.
U.S. Pat. No. 3,393,975 to Mitchell et al. is directed to the production of aluminum sulfate from clays and fly ash, using a multiple-stage countercurrent leaching operation. In this operation, alumina-containing feed is introduced to the first in a series of vessels (e.g., four), and sulfuric acid is introduced to the last vessel. Liquid, comprising progressively weaker sulfuric acid and progressively stronger aluminum sulfate solution, is successively transferred to the preceding vessels of the series. At the same time, solids having a progressively lower aluminum content are successively transferred to later vessels of the series. Liquid which exits the first vessel is concentrated and a solid aluminum sulfate product is recovered by crystallization. In addition to requiring complex equipment, the process of Mitchell et al. suffers from the need for pumping large quantities of abrasive slurry through several operations, and from the slow product filtration step.
A need remains, however, for a treatment which is applicable to the less reactive forms of aluminum oxide, and which permits recovery of the valuable metal components from a spent catalyst while leaving a solution of aluminum sulfate which meets the applicable commercial specifications for a marketable commodity.
Accordingly, it is an object of the present invention to provide a method by which catalysts comprising transition metal compounds on aluminum oxide-containing supports can be treated to recover the various metals and also obtain a useful aluminum sulfate solution.
Another object is to provide a catalyst metal recovery treatment wherein the metals are dissolved in an uncomplicated device which permits close control of product acidity levels.
It is a further object to provide a treatment wherein the product from the dissolving operation contains, at most, a very small amount of insoluble solids, thus requiring little filtration.
A still further object is to treat catalyst materials to obtain a solution from which metals can be extracted to leave approximately stoichiometric equivalents of aluminum and sulfate to form Al.sub.2 (SO.sub.4).sub.3.
These, and other objects and advantages of the invention, will become apparent from the following description and examples.